Our invention comprises improvements in pressure controls for an automatic transmission control valve circuit which is adapted to be used in an electronically controlled transmission for a vehicle with a throttle controlled engine. The circuit includes solenoid valves for controlling transmission shift valves to establish ratio changes so that the torque demand may be satisfied by the internal combustion engine to obtain optimum overall driveline performance while maintaining optimum fuel economy. The ratio changes and the operating pressure level in the control circuit are effected by solenoid valve operators that are under the control of an electronic microprocessor.
Other prior art transmission control systems use an engine throttle valve for establishing a torque signal (TV pressure) that is representative of the engine torque and a fluid pressure governor that is connected mechanically to a torque output member of the transmission for developing a signal that is representative of vehicle speed. In transmissions of this kind, ratio changes to effect desired driveline performance under varying operating conditions are made in response to these torque and speed signals. Examples of control systems of this kind are shown in U.S. Pat. Nos. 4,633,738 and 4,665,770.
Automatic transmissions of this kind normally include a hydraulic control valve system having clutch and brake servos that control relative motion of the planetary gear elements of the transmission to establish selectively in sequence multiple torque flow paths of differing ratios from the engine to the vehicle traction wheels.
A hydrokinetic torque converter is located between the engine and the torque input planetary gear elements. The converter multiplies engine torque during vehicle acceleration and acts as a hydrokinetic coupling during steady state operation. A lockup clutch is provided to bypass the converter during steady state operation in the highest speed ratio in a range of ratios selected by the operator that includes an overdrive ratio. The vehicle operator may select either of two automatic shift ranges, drive range "3" and overdrive range "4", the latter range having an overdrive ratio.
It is common practice also to use in the control system an engine-driven positive displacement pump under the control of a main pressure regulator, the latter responding to changes in the torque signal (TV pressure) made available by the throttle control valve so that the regulated pressure level in the control circuit will increase as torque increases. By controlling the magnitude of the circuit pressure in this way, the line pressure made available to the clutch and brake servos can be tailored during shifts and for other transient operating conditions to eliminate harshness in the engagement of friction torque portions of the clutch and brake servos, thereby reducing the level of harshness perceived by the occupant of the vehicle during normal shifting operations, during tip-in and tip-out shifting, and during ratio changes that occur during coasting and light throttle operation.